1. Field of the Invention
This invention relates generally to electronic article surveillance systems and more particularly, to an article surveillance system that involves the transmission of two distinct radio frequency signals, one of which is tone modulated, that are picked up by a transponder and mixed through a non-linear impedance to be reradiated at a higher frequency equal to their sum, which is detected by a narrow band receiver. This invention is an improvement on the system described in the copending commonly assigned applications of Harold B. Williams, Ser. Nos. 195,572, filed Oct. 9, 1980 now abandoned and 373,251, filed Apr. 29, 1982, now U.S. Pat. No. 4,471,344, issued Sept. 11, 1984. Ser. No. 373,251 is a continuation of Ser. No. 195,572. Both applications are entitled "Dual Frequency Anti-Theft System".
2. Prior Art
Earlier surveillance systems of this type, such as that described in U.S. Pat. No. 4,063,229 to Welsh et al., operate to transmit a single radio frequency to be picked up by an antenna on a transponder tag or label where a non-linear impedance, such as a semiconductor diode, generates a selected harmonic of the transmitted signal that is reradiated for detection by a receiver circuit to the exclusion of the transmitted frequency. However such systems proved unsatisfactory in practice from the standpoint of lacking the sensitivity to reliably detect the presence of a transponder within the surveillance area and of producing false alarms in response to various other conditions, particularly when objects which either strongly reflect radio frequency (RF) signals or behave approximately like the tags in generating harmonics come close to the receiver-transmitter units.
Significantly, the non-linear characteristics inherent in the transmitter circuitry and elements often resulted in harmonics being transmitted along with the fundamental transmission frequency causing the receiver to respond without the presence of a non-linear impedance element in the transponder. If receiver sensitivity has to be reduced to ignore such directly transmitted or reflected harmonics, then lower energy harmonics reradiated by a transponder element which should have been detected under some circumstances might be masked. Although this problem can be minimized by proper shielding and RF filtering in both the transmitter and receiver, the filters would have to be provided with extremely sharp cutoff characteristics so that even a small frequency drift in the transmitted signal, which is multiplied in the harmonic, could easily result in the reradiated frequency being outside of the filter pass band of the receiver. Furthermore in such systems the use of connections between transmitter and receiver to provide the precise reference frequency provides another path for such undesired harmonics to propagate by and that effect is made even greater when a common antenna or closely positioned antennae are used for transmission and reception.
On the other hand, such high frequency signals could readily propagate outside of the intended surveillance area to cause false triggering of the alarm by a remote transponder. As a result, protected articles often could not be located or handled anywhere in the vicinity of the surveillance area. Even then, the high frequency energy might propagate by unpredictable reflections, or even along plumbing pipes or power conduits acting as wave guides, to and from remote locations within the protected structure to produce false triggering of the alarm system.
Such systems were also susceptible to false triggering by metal objects such as umbrellas, baby carriages and shopping carts, where a weld or contact point between dissimilar metals produces a non-linear impedance diode effect to generate and reradiate a harmonic of the transmitted signal. Or the receiver could respond to spurious radio frequency noise from other sources such as motor ignition systems and electronic equipment.
Conversely, the system might not respond to the actual presence of a transponder element within the surveillance area if the energy picked up and reradiated as a harmonic were insufficient. For example, this could occur if the transponder antenna were improperly oriented with respect to the polarization of the transmitted field or if the antenna were to be electromagnetically shielded from the transmitter by the human body or a metallic surface. Also, proximity of the transponder to the human body can detune the resonant tank circuit, thus dissipating the harmonic energy available for reradiation to the receiver. Moreover, although signal tracking circuitry can be incorporated to adjust the frequency response of the receiver to compensate for transmitter frequency drifts, transponder efficiency suffers badly whenever the tuned tank circuit is forced to oscillate at frequencies other than its normal resonant frequency.
Later efforts to resolve the problems of such earlier systems have resulted in several variations. In one of these, which is described in U.S. Pat. No. 3,631,484 to Augenblick, the single radio frequency transmitted to the transponder to be reradiated as a harmonic is compared with signals picked up by the receiver to detect Doppler effect frequency shifts caused by movement of the transponder. Although this system eliminated problems associated with transmitter frequency drift and false alarms from stationary transponders nearby, an article moved slowly through the surveillance area would not produce a Doppler frequency shift sufficient to trigger the alarm.
Attempts were also made to investigate systems wherein the non-linear impedance element in the transponder operated as a signal mixer to generate sum and difference frequencies in response to two transmitted signals of different frequencies, as pointed out in the background discussion of U.S. Pat. No. 3,895,368 to Gordon et al. However, such dual frequency mixer systems were considered to have many practical shortcomings, which included the problem of confining higher frequency transmissions to the intended surveillance area. To overcome this problem, the Gordon et al patent describes use of a dual field system employing a high frequency electromagnetic field in conjunction with a high power, low frequency electrostatic field established between discontinuous conductors disposed on opposite sides of the surveillance space. The non-linear impedance element subjected to these two fields operates as a mixer to produce sum and difference frequencies that are reradiated to the receiver for detection. However, the power required to establish the required electrostatic field within the surveillance area is significant, and such low frequency electrostatic fields can be effectively shielded from the transponder by the human body or by a surrounding conductor and diverted from the transponder through the metallic structure of a shopping cart or the like. Also the low frequency electrostatic field could readily be diverted through nearby pipes and other metal structures to remote locations to cause false triggering by tags far outside the surveillance area, and the problem of false alarms due to dissimilar metal junctions in metal carts and the like was aggravated by concentration of the electrostatic field through such metal structures.